As is known, in the production of extruded products in soft or rigid thermoplastic material of the strip, sheet, pipes or sections, films or flat heads type or similar manufacture, materials are used of the PP (polypropylene), PE (polyethylene), PET (polyethylene terephthalate), ABS (acrylonitrile butadiene styrene) type and the like, which can have contaminating elements or impurities such as iron, wood, paper or textile fibres, small percentages of aluminium or lead or the like, harmful to the good quality and features of the material treated and of the product to be made. This condition is particularly frequent in the case of recycling of materials.
Moreover, in the sector of recycling of industrial reject plastic materials or those coming from differentiated waste collection, it is important to eliminate the impurities completely in order to obtain a material of good quality to be subjected to regranulation.
In order to obtain a material without the aforementioned impurities it is necessary to perform screening of the material being made, performed by means of a screen positioned between the extruder apparatus and the cutting apparatus.
These screens, due to the impurities or contaminants present in the thermoplastic materials, have to be cleaned periodically or preferably replaced and, therefore, are provided with screen changer devices.
The screen changer devices are of different production types as a function of the method of replacement of the screen and of management of the flow of molten material during the same replacement.
Some screen changer devices are actuated manually; this entails the burdensome disadvantage of having to intervene manually in order to be able to proceed with replacement of the screen with a consequent increase in the costs linked to labour, down times and substantially to production.
Other technical solutions provide for the use of automatic systems in which the flow of molten material is temporarily interrupted, accumulating it in a storage cylinder and releasing it when the replacement of the screen has been carried out.
However these solutions, even if they do not need the manual intervention of an operator for performing the operation of replacement of the screen, entail, in any case, a temporary interruption of the flow of material which on some continuous applications, of the sheet or profiles type, cannot be used, with a consequent extension of the times of production/manufacture and correlated costs.
For example EP 0976525 illustrates a complex screening device for plastic particulate material which comprises a body provided with a screening chamber inside whereof are placed transversely a screening band, valves with axial movement controlled and actuated by control means of the actuators type and suitable for enabling/disabling the passage of the flow of plastic material through the conduits and the screening band, means of locking and sealing in place of the screening band at an annular portion of a perforated discoidal element placed inside the screening chamber and, likewise, a cut-off valve co-operating with said valves with axial movement for enabling/disabling the flow of material through the conduits.
U.S. Pat. No. 3,855,126 describes a suitable device for screening of contaminating elements present in a plastic material which comprises an element of containment or chamber, a screening means of the screening band type sliding and placed between an inlet compartment and an outlet one, means of gripping and moving forwards these screening means in the containment chamber and means suitable for ensuring the sealing of the screening band with respect to the chamber so as to avoid possible losses of said contaminating elements. However this device or apparatus has a considerable disadvantage linked to the need to have to interrupt the flow of material during the phase of forward movement of the screening band for the replacement of the same.
U.S. Pat. No. 4,849,113 describes another known device or apparatus suitable for the screening of polymeric materials which, in particular, comprises a cut-off valve suitable for blocking the flow of material through a conduit so as to allow the passage thereof in another conduit and to allow the replacement or moving of a screening band at the conduit closed by the aforementioned valve. However this solution also has some disadvantages linked to the constructional complexity and to the losses of plastic material with consequent increases in the costs of production.
Other automatic screen changer systems provide for the use of cartridge filters provided with a scraper which acts on a micro-perforated lamina placed inside the screen body.
However these solutions too have some important disadvantages linked to the fact that the scrapers tend to limit the screening of the molten material and entail the rejecting of a good percentage of molten material.